1. Field of the Invention
This invention relates to vapor-cooled electrical apparatus and, more particularly, it pertains to a vapor-cooled power transformer.
2. Description of the Prior Art
Existing gas-insulated, vapor-cooled power transformers require a pump to spray insulant onto the core and coils, and at start-up require sulphur hexafluoride (SF.sub.6) gas for insulation, such as disclosed in U.S. Pat. Nos. 3,819,301; 3,834,835; and 2,845,472. A disadvantage of such a system is that it requires a conventional mechanical pump which, comprising moving parts, may incur reliability problems. Also, although SF.sub.6 has a high dielectric strength, its presence reduces the cooling efficiency of the system.
As a result of the foregoing, a need exists for gas-insulated, vapor-cooled transformers that are of comparable efficiency and more fire resistant than conventional oil-filled transformers. The need is particularly opportune because polychlorinated biphenol, which was used as an insulant in many transformers, has been banned due to its non-biodegradable characteristics. In addition, only a small quantity of fluorocarbon, an inert, fireproof, vaporizable liquid, is required for both cooling and insulation in vapor-cooled transformers.
Recirculating systems having a pump are used to continuously spray a liquid coolant onto the windings and core where the coolant vaporizes upon contact. The heavier than air vapors carry off heat into cooling tubes where the vapors condense. The liquid then drains back to a sump from where it is recirculated to the windings. As the transformer load increases, the pressure of the coolant vapor increases which improves the dielectric strength. However, when a vapor-cooled transformer is first switched on, especially at low temperature (&lt;0.degree. C.), depending upon load conditions, there may be a time lag of from 10 to 45 minutes before the dielectric strength of the vapor is adequate. Consequently, SF.sub.6, which has a high dielectric strength, has been added for the initial period of the time lag, but this reduces the cooling efficiency.